What are planets? The planet Venus from space Before there were planets For several hundred million years after the Big Bang, there were no planets, only stars. Planets couldn't form until the first stars ran out of fuel and exploded into supernovas, about 14 billion years ago. The supernovas spewed out thousands of tons of carbon, oxygen, iron and other elements into space. Planets [...]

Main sequence stars: A starry sky with the Milky Way galaxy (thanks to Bernd Nies) What is a main sequence star? Nine out of ten stars that you see in the sky are main-sequence stars, and our sun is one of them. (More about the sun) Most main-sequence stars probably started out as brown dwarfs, and then succeeded [...]

Earth from space: How old is the Earth? When did the Earth form? Like Mercury and Venus and the other planets that go around the Sun, the Earth probably formed a little more than four and a half billion years ago. (That's about 4,600,000,000 years ago.) What's the sun made of? How do planets form? All our physics articles What [...]

Kids playing tetherball using centrifugal force Who first explained centrifugal force? When something is going straight, it always keeps going straight unless something else stops it or turns it. (The Iranian scientist Ibn Sina figured this out about 1000 AD). Who was Ibn Sina? Medieval Islamic science All our physics articles An example of centrifugal [...]

This is a sphere. A sphere is a three-dimensional circle. Or you could say that a sphere is the set of all the points that are at the same distance from the center of the sphere. In nature, centrifugal force and gravity tend to make a lot of things into spheres: soap bubbles, for instance, atoms, and planets. The radius of a sphere [...]

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